1. Field of the Invention
The present invention relates to a decompression device in a two-cycle engine which can reduce a starting torque upon start of the engine.
2. Description of the Prior Art
One prior art example of a two-cycle engine having decompression means is illustrated in FIG. 7. The illustrated engine is a piston valve type spark-ignition two-cycle engine. In this figure, reference numeral 1 designates a cylinder, numeral 2 designates a cylinder liner forming an inner wall of the cylinder 1, numeral 3 designates a piston, numeral 4 designates a cylinder head, numeral 5 designates a combustion chamber, numeral 6 designates an exhaust port, and an opening 6a of the exhaust port 6 on the side of the cylinder inner wall is opened and closed by sliding movements of the piston 3.
On the inner wall of the above-mentioned cylinder 1 is scooped out or formed a compressed gas leak groove 7 located within the range of and adapted to be opened and closed by sliding movements of the piston 3. Groove 7 extends from the top edge of the opening 6a of the exhaust port 6 on the side of the cylinder inner wall towards the upper dead point as shown in FIG. 8.
This compressed gas leak groove 7 is formed in an inverse isosceles trapezoid shape in a plan configuration having a portion 7a communicating with the opening 6a of the exhaust port on the side of the cylinder inner wall side choked into a narrow width as compared to its top starting edge portion 7b, as shown in FIG. 9.
It is to be noted that examples of modifications of the above-described compressed gas leak groove 7 of inverse isosceles trapezoid shape are a compressed gas leak groove 8 of oval shape in plan configuration as shown in FIG. 10, a compressed gas leak groove 9 of T-shape as shown in FIG. 11, a compressed gas leak groove 10 of inclined key hole shape as shown in FIG. 12, and a compressed gas leak groove 11 of X-mas tree shape as shown in FIG. 13. End portions 8a, 9a, 10a and 11a of such grooves communicating with the opening 6a of the exhaust port 6 on the side of the inner wall of the cylinder are formed to be narrow in width.
Also, in some cases, there may be provided plural of the above-mentioned compressed gas leak grooves 7-11 distributed along the opening 6a of the exhaust port 6 on the side of the inner wall of the cylinder. Furthermore, the top starting end portion of each of the above-described compressed gas leak grooves 7-11 is disposed preferably at a position of 40.degree. or less in a crank shaft angle.
Still further, preferably the above-mentioned compressed gas leak groove 7 (or 8-11) is formed in such manner that the depth of the groove at the top starting end portion 7b is relatively shallow, but that the depth at the communicating portion 7a is deep, resulting in an advantage that flow of the exhaust gas passing through the compressed gas leak groove 7 is made smooth. It is to be noted that reference numeral 13 designates an air feed port.
In the above-described prior art decompression device in a two-cycle engine, the decompression passageway (compressed gas leak groove) 7 communicates with the exhaust port 6 and the combustion chamber 5.
Accordingly, an incomplete combustion gas that is inherent to a two-cycle engine will pass through the decompression passageway 7 and will escape through the exhaust port 6. At this time, carbon soot is liable to block the decompression passageway 7, and thus the function of the passageway is deteriorated.
Furthermore, regarding the process for manufacturing the engine, in order to provide the groove 7 in the prior art, an inner mold of a cylinder would be withdrawn in a direction opposite to a plug. Accordingly, at first a mold for forming the groove 7 must be moved to a central portion, and in the subsequent step of the process the inner mold must be withdrawn downwards, so that man-hours required for the manufacturing operation is increased.
In addition, if the groove is provided in the above-described manner, there is a disadvantage that reduction of output power results due to lowering of a compression pressure of the engine and due to leakage of gas through an escape groove upon an expansion stroke after ignition.